Radiocircuit



Aug. 28, 1934. 1.. JONES El AL RADIOGIRCUIT Fild Oct. 29,

1929 2 Sheets-Sheet 1 AUTOMATICALLY TUNED AMPLIFIER I so LINK

AUTOMATICALLY TUNED AMPLIFIER LINK INVENTORS LESTER L.JONE$ JACOBYOLLES- BY ATTORN Y5 Aug. 28, 1934. L. L. JONES ET AL 1,971,487

RADIOCIRCUIT Filed Oct. 29, 1929 2 Sheets-Sheet 2 -Patented Aug. 28,1934 .PATEN'F mmocmourr Lester L. Jones Oradell, J., and Jacob Yolles,

Brooklyn, N. Y., assignors to Technidyne Corpcratron, New York, N. Y., acorporation of New York Application October 29, 1929, Serial No. 403,161

23 Claims.

This inventionfrelates to radio circuits and more particularly to suchcircuits including amplifiers having fixed circuit constants.

In a patent to Lester L. Jones No. 1,673,287, issued June 12, 1928, hediscloses how a vacuum tube amplifier stage may have fixed output andinput circuits so relatively adjusted that the output circuit serves tokeep the input circuit of the tube in tune with the frequency of energybeing sent through the amplifier. The output circuit there disclosedworks into a detector tube which, owing to the output circuit thereofbeing shunted for radio frequencies, has a small fixed input capacitancewith respect to radio frequency energy. This permits the output circuitof the automatically tuned stage to be kept resonant to a fixedfrequency lower than the working range of frequency, as is desirable forautomatically tuning the input circuit of the amplifier stage.

If it be desired to have a plurality of similar amplifier stagesarranged in cascade it is not feasible merely to cascade such stagesdirectly together because, for example, the input circuit of the laststage, which acts as the output circuit of the next to the last stage,instead of remaining resonant to a constant low frequency, is kept intune with the working frequency and therefore is not suitable forautomatically tuning'the input circuit of the next to the last stage.Ina copending application of Lester L. Jones, Serial No. 397,633, filedOct. 5, 1929, since issued as Patent 1,779,881, dated October 28, 1930,he has ex plained how a plurality of automatically tuned stages may becascaded directlytogether by arranging each of the stages to tune overdifferent portions of the'frequency range and'cascading the stagesdirectly together in the order of decreasing frequency.

This arrangement has an advantage when dealing with a broad frequencyrange because it is difficult to provide a stage which will tuneautomatically over a very broad frequency range while employing ordinarytubes. However, it is not desirable to intentionally subdivide the ire--quency range into a greater number of frequency portions than isnecessary'to cover the desired range merely inorder to cascade a greaternumber of amplifier tubes. V Y

The primary object oft'n'ef present invention is to make possible theuse oi'any desired number of automatically tuned stages regardless ofthe breadth of the frequency range and regardless of the frequency rangeto which any one stage is capable offtuning. For-this purpose we have 55found it possible to provide-between two automatically tuned amplifiersa specialfcrm of coupling stage, which, for the of convenience andbrevity, We shall herein refer to as a link stage or link circuit. Such'a link stage may and preferably does include a tube having at all timesa constant apparent input capacitance, into which capacitance the firstor preceding automatically tuned amplifier works. This constantcapacitance makes it possible for the input circuit of the link stage,which operates also as the 5 output circuit of the first amplifier, tobe made naturally resonant to a constant frequency lower than thedesired range of frequency, which is the most favorable condition forautomatically tuning the first amplifier. By the use of such linkstages, if a single automatically tuned stage is capable of covering thedesired frequency range anda powerful amplifier is needed, it is merelynecessary to cascade. automatically tuned stages and link stages inalternation. If the frequency range is broad a plurality ofautomatically tuned stages may be cascaded directly together to form asingle automatically tuned amplifier capable of covering the desiredfrequency range, in accordance with the teachings of the copendingapplication Serial No. 397,633, since issued as Patent 1,779,881,aforesaid, and, if a more powerful' amplifier is needed, it is merelynecessary to employ link stages for coupling several such completeamplifiers in cascade.

A further object of our invention resides in the provision of a suitablearrangement or circuit to act as the so-called link circuit or linkstage for the foregoing purpose. lhis may be done with an ordinary tubeby so designing the output circuit of the tube as to prevent reactionupon the input circuit of the tube and to obtain a small constantapparent input capacitance for the tube.

It may also be accomplished by the use of a special tube such as thescreened grid tube, in which the constant desired input capacitance isobtained by reason of a fourth electrode which shields the controlelectrode from the anode of the tube. L

A still further object of the present invention resides in'the provisionof a complete receiver embodying the foregoing features of my invention,and further embodying a novel and relatively simplified link circuit forpreventing reaction upon a sharply tuned pre-selector byan amplifierfollowing the selector.

To the accomplishm nt of the foregoing and such other objects as willhereinafter appear, our invention consists in the circuitelements andtheirrelation one to the other as, hereinafter are more particularlydescribed in the specification and sought to be defined in the claims.The specification is accompanied by drawings in which:

Fig. 1 is a wiring diagram showing automatically tuned amplifier stagesworking into a detector tube;

Fig. 2 is a schematic diagram explanatory of ou invention;

Fig. 3 is one form of link circuit which may be used betweenautomatically tuned amplifiers;

Fig. 4 is an alternative form of link circuit which may be used betweenautomatically tuned amplifiers;

Fig. 5 is a differently coupled form of link stage which may be usedbetween automatically tuned amplifiers;

Fig. 6 is a schematic wiring diagram for a complete receiver embodyingour invention;

Fig. 7 is a wiring diagram for a modified amplifier employing linkstages similar to that in Fig. 3; and

Fig. 8 is a wiring diagram for another amplifier employing link stagessuch as are shown in Fig. 4.

Referring to Fig. 1, there is an amplifier tube 2 followed by a detectortube 4. The output circuit of the detector tube is provided with afilter section 6 for separating the radio frequency and modulationcomponents of the received energy, and this being a low impedance shuntfor radio frequencies, causes the input circuit of detector tube 4 tohave an apparent input capacitance for radio frequencies represented bythe dotted condenser 8, which is constant and pure.

The amplifier tube 2 is provided with an input transformer 10 and anoutput transformer 12, both of which are preferably bifilarly wound soas to have substantially unity coupling. The transformer 12 is madenaturally resonant in circuit F to a frequency lower than the lowestfrequency of the working frequency range. For frequencies in the workingfrequency range the output circuit of tube 2 is therefore capacititveand decreases in impedance with increasing frequency, causing decreasingapparent input capacitance with increasing frequency in tube 2, thiscapacitance being represented by the dotted variable condenser 14.Transformer 10 is selected to resonate with variable capacitance 14 forautomatic tuning.

Variation of capacitance 14 is obtained because the output circuit oftube 2 remains naturally resonant to a low frequency, and this in turnis made possible by capacitance 8 remaining fixed. At high frequenciestransformer 12 departs from resonance and is low in impedance, whichreduces the apparent input capacitance 14. The range of tuning may beincreased in accordance with the teachings of our copending applicationSerial No. 402,378, filed October 25, 1929, since issued as Patent1,909,968, dated May 23, 1933, by the use of a series inductance section16 so dimensioned as to series resonate with the input capacitance atthe high frequency end of the range. The resulting shunt acrosstransformer 12 still further reduces the impedance thereof at highfrequencies and still further reduces the apparent input capacitance 14,and thereby increases the available range of automatic tuning. The gainat high frequencies is also increased by the resonant rise in potentialobtained across the capacitance 8 in the series resonant circuit.

If amplifier tube 2 is preceded by another amplifier tube 20 it is notpossible to make the tube 20 tune automatically over the same frequencyrange as the tube 2. This follows bethe frequency range.

cause the output circuit of tube 2, instead of being naturally resonantto a constant frequency lower than the frequency range, is resonant to avariable frequency and remains in tune with the received frequency. Theresulting high impedanee causes the apparent input capacitance of tube20, represented by the dotted condenser 22, to remain high, andtherefore is not suitable for automatically tuning the input circuit oftube 20.

- This stage differs from the stage including tube 2,

the output circuit of which, corresponding to input circuit of thedetector tube 4, remains naturally resonant to a constant frequencylower than In referring to the resonance frequency of the input circuitof detector tube 4 the frequency for parallel resonance is meant. Theseries resonance of inductance 16 and capacitance 8 at high frequenciesis really equivalent to an increase in the departure of the circuit fromparallel resonance at high frequencies, in that the series resonantshunt further reduces the impedance of transformer 12 at highfrequencies.

In accordance with the disclosure of the copending application SerialNo. 397,633, since issued as Patent 1,779,881, already referred to, thetubes 20 and 2 may be cascaded directly together when dealing with abroad range of frequency provided that tubes 2 and 20 are adjustedrespectively to automatically tune over the lower frequency portion andthe higher frequency portion of the frequency range, because the highestfrequency to which the input circuit of tube 2 can then tune is anintermediate frequency, and for the still higher frequencies in the highfrequency portion of the range the input circuit of tube 2,corresponding to the output circuit of tube 20, is resonant to arelatively lower frequency and therefore is suitable for automatictuning. It then would not be possible to cascade a third automaticallytuned stage ahead of the tube 20 unless the working frequency range weredivided into three different frequency portions. However, it is notdesirable to do so unless such a step proves necessary in order to coverthe desired frequency range.

In accordance with the present invention an automatically tunedamplifier, such as the amplifier 30 in Fig. 2, which may consist of oneor as many more stages as may be needed to cover the desired frequencyrange, may be preceded by another automatically tuned amplifier, 32,which may, if desired, be a similar amplifier arranged to cover the samefrequency range, provided that the amplifiers 30 and 32 are coupledtogether by a special link such as the link 34, which in one aspect maybe considered as being characterized by a fixed and prefectly pureapparent input capacitance, and which may be considered in anotheraspect, as having an input circuit which is naturally resonant to aconstant frequency lower than the working frequency range.

Fig. 3 shows one form which the link stage may take. In Fig. 3 there isan electron emission tube 40, the output circuit of which is arranged inaccordance with the disclosure in a patent to Lester L. Jones No.1,713,132, issued May 14, 1929, which may further embody theimprovements set forth in his copending application Serial No. 397,632,

filed October 5, 1929, since issued as Patent r erative in the workingfrequency range. Resistance 44 is selected to provide a feedforward ofenergy through tube 40 for neutralizing the'feed back caused byinductance 42. The succeeding circuit may be coupled to the outputcircuit so far described by a transformer 46 arranged in parallel withthe output circuit. Various changes may be made in this circuit, theeffect of which is described in the said patent andcopendingapplication, but generally speaking; when the output circuit is properlyarranged and. propor tioned the effect thereof is to prevent reactionthrough the tube 40 and to cause the tube to have an apparent inputcapacitance, indicated-by the dotted condenser 48, which issubstantially fixed. This permits the selection of an input transformer50 which is naturally resonant in circuit to afrequency lower than theworking frequency range. The conductance and capacitance valuesremaining constant, the resonance frequency will be fixed, as is desiredfor automatically tuning a preceding amplifier stage.

In Fig. 4 we have illustrated a different form of link circuit in whicha screened grid vacuum tube 52 is employed, having in its output circuita transformer 54, selected as the desired transformer for a succeedingautomatically tuned amplifier. The control electrode and anode of thetube are separated or shielded by a screen 53, and the apparent inputcapacitance of the tube,'repre sented by the dotted condenser 56, istherefore fixed, and consequently a bifilar wound input transformer 58may be selected which will be naturally resonant in circuit to aconstant frequency lower than the working frequency range,

suitable for the output circuit of a preceding automatically tunedamplifier stage.

In Fig. 5 we have illustrated a very similar form of link circuit inwhich, however, a form of impedance coupling rather than transformercoupling is used. As inthe link circuit shown in Fig. 4, there is ascreened grid vacuum tube 252 having in its output circuit a transformer254 selected as the desired transformer for a succeeding automaticallytune amplifier. The control electrode and anode of the tube areseparated or siielded by a screen 253 and the apparent input capacitanceof the tube, represented by the dotted condenser 256, is thereforefixed, and consequently an inductance 258 may be selected which will benaturally resonant in circuit, allowing for the effect of inductance259, to a constant fre-'- quency lower than the working frequency range.The output circuit of the preceding automatical- 1y tuned amplifierincludes the inductance 259' input circuit of the link staga but whichat the same time blocks'the high direct anode potential of the precedingtube from reaching the control electrode of the link tube 252. t 4

A complete receiver embodying our invention is illustrated in Fig. 6 ofthe drawings. This receivercomprises a suitable antenna circuit 60, theenergy pick-up being fed to a sharply tuned selector 62 having variablecircuit constants. Selected energy of desired frequencyis amplified in aplurality of automatically tuned amplifiers 64, 66 and 68, The selectorand the automatically tuned amplifiers are connectedin cascade by linkcircuits "70, '72 and 74, which may incidentally further amplify theselected energy. The. radio frequency and modulation components areseparated by means of a detectorcircuit76, after which the modulationcomponent mayif desired, be furtheramplified, and translated in anysuit-- able reproducer.

For preventing reaction by the automatically tuned amplifiers on theselector we employ a link circuit 70 of a type which may be organizedfor better efficiency than that set forth in the 'copending applicationSerial No. 397,632, sincev issued as Patent 1,788,197, alreadymentioned, disclosing reactionless link means for preventing reaction bythe amplifier on the selector. The present link circuit includes ascreened grid tube 80, and the input circuit of the'tube is connected tothe last tune circuit,'82, of the selector, While the output circuit ofthe tube includes a transformer 84, the magnitude of which is selectedto be automatically tuned by the input capacitance of the first tube inthe automatically tuned amplifier 64. Transformer 84 is preferablybifilarly wound, and may consist of two sections wound in oppositedirections in order to make the transformer 'astatic, in accordance withthe patent to Lester L. Jones, No. 1,732,937 issued October 22, 1929 I IAutomatically tuned amplifier 64 may consist of a single amplifier stageor of a plurality of amplifier stages coupled directly in cascade andarranged to tune to different portions of the working frequency range.

It is coupled to automatically tunedamplifier 66 by means of a linkcircuit '72, which in this case has been illustrated as similar to thatshown in Fig. 3, but which may be like that'snown in Fig; 4 of thedrawings. The input transformer 86 is selected to be naturally. resonantin circuit to a frequency lower than the frequency range. It ispreferably bifilarly wound in four alternately oppositely woundsections, in accordance with the copending application of Lester L.Jones, Serial No. 402,379, filed October 25, 1929, since issued asPatent 1,901,368, dated March 14', 1933. A series inductance 88 may beconnected in series with the input circuit of thelink tube, theinductance being of suitable magnitude to series resonate with theapparent input capacitance of the tube at the high frequency end oftherange, like inductance 16 in Fig. l, to which the induc-v tance 88corresponds both in location and in operation. The output transformer 90is a bifilar astatic transformer of suitable magnitude to beautomaticallytuned by the input capacitance of the first tube in theamplifier 66.

Link circuit-74 is similar to that illustrated in Fig. 4, "andinclud esa screene'dgri'd tube. The

"with the apparent input capacitance of the tube at the highfrequenciesfl The output transformer 96 is selected to be automaticallytuned by the input capacitancejofthefirst tube in the arn- "The detectorcircuit "ifi' includes in its output afilte'r section 98*for separatingthe radio frequency and modulation components of the rectified carrier.The resulting low impedance shunt for radio frequencies causes the inputcircuit of the detector tube to have a fixed input capacitance, andtransformer 100 is selected to be naturally resonant in circuittherewith to a frequency lower than the working frequency range. Aseries inductance 102 may be used, as was explained in connection withFig. 1.

It will be understood that while three automatically tuned amplifiershave been shown in the present case, less than three or more thanthree'may readily be employed, depending upon the total gain desired inthe amplifier. It will also be understood that each of the automaticallytuned amplifiers may consist of one or more stages, and that when morethan one stage is used in each amplifier said stages may be cascadeddirectly together by tuning them to different portions of the frequencyrange. It will further be understood that each of the automaticallytuned amplifiers may have the same number of stages or different numbersof stages. For example, suppose there are two amplifiers each consistingof a single stage, and that the total gain is slightly less than isdesired, so that the addition of another link stage and anotherautomatically tuned amplifier increases the total gain far more than isdesired. In such case one of the amplifiers may be made a two-stage ameplifier, each stage being tuned over a portion of the frequency range.

In another case there might be two amplifiers each having two stages.For a slight increase in overall gain one of the amplifiers may be madewith three stages. For a further increase in overall gain bothamplifiers might be made in three stages. For a still further increasein overall gain both amplifiers may be made with two stages and a thirdamplifier and link stage added.

Commercially successful, efiicient, and stable automatically tunedamplifiers covering a broad frequency band may be made without the useof intermediate link stages, following the teachings of the copendingapplication Serial No. 397,633, since issued as Patent 1,779,881,aforesaid, but there is great difiiculty when a considerable number ofstages are to be used, and particularly when dealing with tubes having arelatively high anode to cathode impedance, such as dry cell or batterytubes having a small power consumption. In Fig. 7 I have illustrated asix stage amplifier embodying my present invention and using batterytype tubes, for example, tubes having three volt thoriated tungstenstraight filaments.

The amplifier is split into two sections, the first consisting of a pairof tubes T1 and. T2 and the second consisting of the four tubes T3, T4,T5 and T6. The input and output circuits of tube T2 are related in anapproximate way for automatic tuning, the input transformer L2 beingheld generally resonant to the received frequencies by the risingcapacitance with decreasing frequency of the input circuit of tube T2.The transformer L2 tunes very broadly, of course, due to the resistanceR1 shunted across the primary thereof. The capacitance characteristic ismade suitable for correcting the reaction characteristic of the firsttube, and it is primarily to best accommodate this use of tube T2 thatthe first section of the amplifier employs only a single automaticallytuned stage. Resistance R1 is shunted by a small condenser C1 forfurther correction of the reaction characteristic, these corrections allbeing described in the copending application Serial No. 397,632, sinceissued as Patent, 1,788,197, aforementioned. The output circuit of tubeT3 is similar in nature to that of the first output circuit, producing arelatively low and constant input capacitance in tube T3.

It is not necessary to have the same degree of refinement in theelimination of reaction on the input circuit of tube T3 as on the inputcircuit of tube T1. This is because the input circuit of tube T1 wouldusually be the last tuned circuit of a selector, which is a very sharplytuned circuit, whereas the input circuit of tube T3 is broadly tuned,and, therefore, not susceptible to slight reactions through tube T3.Because of this the resistance R2 in the output circuit of tube T3 maybe made'of a-different, and to increase the gain in tube T3, ofpreferably a higher value than that used for the output circuit of tubeT1. It may be increased about two-fold, and a substantiallyproportionate increase also made in the magnitude of the coil L5.

This corresponding increase keeps the regenerative reaction due to thecoil L5 substantially equal to the feed forward action of the resistanceR2. The total increasein the magnitude of the impedance of the outputcircuit of tube T3 is desirable in order to improve the gain therein,and the deviation from complete elimination of reaction over thefrequency range is tolerable on account of the broad tuningcharacteristics of the input circuit of tube T3, as aforesaid. Thecapacitance in shunt with the resistance R1 may in the case ofresistance R2 also be eliminated, since its function is to correct thereaction characteristic to a degree of refinement which is unimportantin tube T3.

The inductance L4, included in series with the grid of tube T3, as inthe case of the similar inductances in Fig. 6, series resonates with theinput capacitance of tube T3 at approximately the highest frequency ofthe band to be amplified. The second section of the amplifier can bestbe considered starting back at the detector tube T7. This tube has aslightly smaller input capacitance at radio frequencies than tube T3because its output circuit is effectively shunted for radio fre-- quencycurrents. In consequence both the transformer inductance L9 and the freegrid section L10 are slightly larger than the corresponding elements inthe input circuit of tube T3. The output circuit of tube T6 is arrangedto give to tube T6 an input capacitance characteristic suitable fortuning the input transformer L8. Transformer L8 is arranged toautomatically tune transformer L7, and the latter is arranged toautomatically tune transformer L6, but this automatic tuning is overstaggered portions of the frequency band, all as is described supra andin the copending application Serial No. 397,633, since issued as Patent1,779,881, previously mentioned.

The introduction of the reaction free link stage including tube T3 isextremely effective in smoothing out the gain curve of the completeamplifier. Also, the reaction characteristic of the Whole amplifier uponthe selector is made more closely adherent to the normal desiredreaction free characteristic, because the intermediate reaction freelink circuit acts as a buffer against the transmission of reaction aswell as capacitance changes from the last amplifier stages back throughto the first amplifier stage.

Suitable quantitative design constants are next given, assuming thattheamplifieris intended to operate over a frequency range of from about500 to 1500 kilocycles, and that battery tubes are to be used of thethoriated tungsten straight filament type, having a grid to platecapacitance of say 3 micro-microfarads, a tube conductanceof say 650micromhos measured with an anode potential of 90 volts and no grid biasother than that obtained by connecting the grid to the nega-' tive sideof the filament, and an amplification factor of about 14. Resistance R1may be 2800 ohms. Capacitance C1 is 20 micro-microfarads. Inductance L1is 285 microhenrys. Transformer L2 is a befilar transformermade astaticin two oppositely wound sections having a total inductance of 1.3millihenrys. v i

The input capacitance of tube T3 with constants as given and with ananode potential of about 100 volts is of the order of 22 micro-micro-.

farads. Inductance L4 is selected to series resonate with this inputcapacitance at a frequency of 1400 rather than 1500 kilocycles in orderto better take advantage of the obtainable resonant rise in potential.The value of inductance L4 therefore is made 0.6 millihenrys. tance oftransformer L3 togetherwith the inductance L4 connected in seriestherewith should resonate in circuit with the input capacitance of tubeT3 at about 550 kilocycles, but there is an appreciable capacitance inparallel with transformer L3 due to the plate to grid and filamentcapacitance of the preceding tube T2, which shunt capacitance alters theeffective value of the inductance of the transformer L3. The effectiveinductance value should be proper for the desired parallel resonance,rather than the actual inductance value of transformer L3 when notconnected in circuit. The value of L3works out to "be 2.77 millihenries,obtained by a two section astatic bifilar wound transformer. IResistance R2 is 5600 ohms. InductanceLS 500 microhenrys. Transformer L6is a bifilar transformer madeastatic in two oppositely'woundsectionshaving a total inductance of 1.3 1nilli-. henrys. It shouldbenoted that. this valueis eifecs tively considerably reduced bythecircuit R2, L5 *45 connected in shunt therewith. Transformer L7 is abifilar transformer made astatic by two oppositely wound sections havinga total inductance of 0.72 millihenrys. Transformer L8 is a bifilartransformer. made astatic in two oppositely wound sections having atotal inductance of 0.86 millihenrys. Transformer L9 is a bifilartransformer made astaticin four alternately oppositely wound sectionshaving a total inductance of 3.6 millihenrys. Coil L10 has an inductanceof 0.75 milli henrys.

-The bypass condensers'C2, C3 and C4 are all about mid. in capacitance,and are constructed for extremely low radio frequency impedance, inorder to reduce distant stage feed back'through common impedance in theB supply and filament circuits. 1 Attention is now directed to theamplifier a wiring diagram forwhich is given in Fig; 8. This amplifieremploys screened grid tubes T1 and T3 for the link circuits. Tube T3separates the amplifier into two automatically tuned sections, one ofwhich has a single stage, T2, and the, other of which has two stages, T4and T5, thisparticular grouping of tubes having been selected in orderto obtain a specified overallgain.

A selector is coupled to the input circuit of the screened grid tube T1.A radio frequency bypass condenser 110 is provided, the function ofwhich is, in cooperation with the radio frequency choke 112, to preventaradio frequency feedback The induc from 'the amplifier to the selectorthrough the cathode leads. I

Transformers 114 and 116 are suitably related to obtain automatic tuningof the input circuit oftube T2. Inductance 118 is preferably connectedin series with the control electrode of the screened grid link tube T3for reasons previously explained. Transformer 120 is automatically tunedover the higher frequency portion of the working frequency range bytransformer 122, and the latter is automatically tuned over the lowerfrequency portion of the working frequency range by transformer 124;Inductance 126 is preferably connected hi series with the controlelectrode of-the detector T6, in a manner similar to that of inductance118 of the link tube T3. It will be appreciated that the link tubes T1and T3, while respectively used primarily to prevent reactiontherethrcugh, and to obtain a constant apparent input capacitancesuitable for automatic tuning, are nevertheless capable of providing anddemovide considerable amplification gain, and servea useful function asamplifier tubes alone, although in this'respect their efficiency isperhaps less thanmight be obtainable were they designed primari-. ly asamplifier tubes. I Design dimensions follow, assuming the tubes T1 andT3 to be alternating current screened grid tubes commercially designatedtype 224, and the tubes T2,"Te and T5 to be alternating current heatertype tubessimilar tothose commercially designated as type 227, exceptthat they have a greater amplification constant, say 12. Transformer3.1a is a bifilar transformer made astatic in two oppositely Woundsections having a total inductance of 0.81 millihenrys. Transformer 116is a bifilar transformer made astatic infour alternatelyoppositely woundsections having a total inductance of 4.0 millihenrys. Inductance .118is made astatic in two oppositely wound sections havinga" totalinductance of one millihenry. Transformer 120 is a bifilar transformermade astatic in two oppositely wound sections having atotalinductan'ceof 0.72 millihenrys. Transformer 122 is abifilar transformer madeastatic in two oppositely wound sections having a total inductanceof1.39 millihenrys. TransformerlZd is abifilar transformer made astatic infour alter natelyoppositely wound sections having a total inductanceof3.0 millihenrys. Inductance 12;6fis made astatic in twooppositely-wound sections having a total inductance of one millihenry."

In general, it will be apparent from the foregoing description that'bythe useof our inven-. tion great freedom and flexibility is obtained inthe design of automatically tuned amplifiers and in the design of radioreceivers embodying the same. "Any desired gain may be obtained whileusing; automatidtuning over any desired range, and while" retaining goodefficiency and a relatively high gain perstage. Automatically tunedamplifiers maybe cascaded indefinitelywithout excessive reaction andwithout limitation by reason of the particular frequency range to beamplified." Y

It will "be apparent that while we have shown and described ourinvention in preferred forms, many changes and modifications maybe madein the circuits disclosed without departing from the spirit of theinvention, defined in the following claims. In manyof these claims, weemploy the term link stage or link circuit, .by which we mean merely astage or circuit having primarily a linking or coupling function,although it may secondarily have other functions. The claims do not relyon the word link for novelty, and include the same not by Way oflimitation, but solely for the convenience of the reader, in order toclarify and better fix in mind the idea of the means comprehended by theclaims.

In all of the claims reference is made to amplifiers which areautomatically tuned, by which we mean self-tuning amplifiers havingphysically fixed circuit elements, in contradistinction to amplifiershaving physically variable condensers or/and physically variableinductances. By automatically tuned we do not refer to that class ofamplifier having variable condensers or coils together with automaticmechanism for varying the same, but rather an amplifier of the classdescribed in the foregoing specification and in the prior patentsthereinreferred to.

We claim:

1. The combination with an automatically tuned radio frequency amplifierof an additional radio frequency amplifier stage coupled thereto andincluding a tube so arranged that the apparent input capacitance thereofis constant.

2. The combination with an automatically tuned radio frequency amplifierarranged to tune over a desired range of frequency of an additionalradio frequency amplifier stage coupled thereto and so arranged that theinput circuit thereof is resonant to a constant frequency lower than thedesired range of frequency.

3. The combination with an automatically tuned radio frequency amplifierof a screened grid tube coupled thereto and into which said amplifierworks, said screened grid tube being arranged in circuit to act as anamplifier tube.

4. The combination with an automatically tuned radio frequency amplifierarranged to tune over a desired range of frequency of a screened gridtube having an input circuit coupled to said amplifier and resonant to aconstant frequency lower than the desired range of frequency, into whichinput circuit said amplifier Works.

5. The combination with an automatically tuned radio frequency amplifierof an additional radio frequency amplifier stage coupled thereto andcomprising a tube having an input circuit into which said automaticallytuned amplifier works, and an output circuit so arranged that theapparent input capacitance of the tube is kept constant.

6. The combination with an automatically tuned radio frequency amplifierarranged to tune over a desired range of frequency of an additionalradio frequency amplifier stage coupled thereto and comprising a tubehaving an input circuit into which said automatically tuned amplifierworks, and an output circuit so arranged that the apparent inputcapacitance of the tube is kept constant, said input circuit beingnaturally resonant to a constant frequency lower than the desired rangeof frequency.

7. A radio circuit comprising an automatically tuned amplifier, a secondautomatically tuned amplifier, and a link circuit for coupling the firstand second amplifiers in cascade, said link circuit including a tubehaving a constant apparent input capacitance into which the firstautomatically tuned amplifier works.

8. A radio circuit comprising an automatically tuned amplifier arrangedto tune over a desired range of frequency, a second automatically tunedamplifier arranged to tune over the same range of frequency, and a linkcircuit for coupling the first and second amplifiers in cascade, saidlink circuit including a tube having a constant apparent inputcapacitance into which the first automatically tuned amplifier works.

9. A radio circuit comprising an automatically tuned amplifier arrangedto tune over a desired range of frequency, a second automatically tunedamplifier arranged to tune over the same range of frequency, and a linkcircuit for coupling the first and second amplifiers in cascade, saidlink circuit having a tube and an input circuit therefor naturallyresonant to a constant frequency lower than the desired range offrequency, into which input circuit the first automatically tunedamplifier works.

10. A radio circuit comprising an automatically tuned amplifier, asecond automatically tuned amplifier, and a link circuit for couplingthe first and second amplifiers in cascade, said linkcircuit including ascreened grid tube.

11. A radio circuit comprising an automatically tuned amplifier arrangedto tune over a desired range of frequency, a second automatically tunedamplifier arranged to tune over the same range of frequency, and a linkcircuit for coupling the first and second amplifiers in cascade, saidlink circuit including a screened grid tube.

12. A radio circuit comprising an automatically tuned amplifier arrangedto tune over a desired range of frequency, a second automatically tunedamplifier arranged to tune over the same range of frequency, and a linkcircuit for coupling the first and second amplifiers in cascade, saidlink circuit including a screened grid tube having an input circuitnaturally resonant to a constant frequency lower than the desired rangeof frequency, into which input circuit the first amplifier works.

13. A radio circuit comprising an automatically tuned amplifier, asecond automatically tuned amplifier, and a link circuit for couplingthe first and second amplifiers in cascade, said link circuit includinga tubehaving an input circuit into which the first automatically tunedamplifier works, and an output circuit so arranged as to cause the tubeto have a constant apparent input capacitance.

, 14;. A radio circuit comprising an. automatically tuned amplifierarranged to tune over a desired range of frequency, a secondautomatically tuned amplifier arranged to tune over the same range offrequency, and a link circuit for coupling the first and secondamplifiers in cascade, said link circuit including a tube having aninput circuit into which the first automatically tuned amplifier works,and an output circuit so arranged as to cause the tube to have aconstant apparent input capacitance.

15. A radio circuit comprising an automatically tuned amplifier arrangedto tune over a desired range of frequency, a second automatically tunedamplifier arranged to tune over the same range of frequency, and a linkcircuit for coupling the first and second amplifiers in cascade, saidlink circuit including a tube having an input circuit into which thefirst automatically tuned amplifier works, and an output circuit soarranged as to cause the tube to have a constant apparent inputcapacitance, said input circuit being naturally resonant to a constantfrequency lower than the desired range of frequency.

16. A radio circuit comprising a sharply tuned selector including aplurality of resonant circuits coupled directly in cascade and havingvariable circuit constants, a radio frequency amplifier including aplurality of stages coupled in cascade and having fixed circuitconstants, and a link circuit for coupling the amplifier to theselector, said link circuit including screened grid tube for preventingreaction by the amplifier on the selector.

17. A radio circuit comprising a selector having variabie circuitconstants, an automatically tuned radio frequency amplifier having fixedcircuit constants, and a link circuit for coupling the automaticallytuned amplifier to the selector, said link circuit including a screenedgrid tube for preventing undesired reaction by the automatical ly tunedamplifier on the selector.

18. A radio circuit comprising a selector having variable circuitconstants, a plurality of automatically tuned amplifiers having fixedcircuit constants, anda plurality of link circuits connecttunedamplifiers in cascade, said link circuits each including a tube arrangedto have a constant apparent input capacitance into which the precedingcircuit works.

20. Aradio circuit comprising a selector having variable circuitconstants, a plurality of automatically tuned amplifiers having fixedcircuit constants arranged to tune over the same desired frequencyrange, and a plurality of link circuits connecting the selector and theautomatically tuned amplifiers in cascade, said link circuits eachincluding a tube arranged to have a constant apparent input capacitanceinto which the preceding circuit works, the last tuned circuit of theselector acting as the input circuit or the first link circuit, and theinput circuits of the remaining link circuits being resonant to aconstant frequency lower than the desired range of frequency.

21. A radio circuit comprising a selector having variable circuitconstants, a plurality of automatically tuned amplifiers having fixedcircuit constants, and a plurality of link circuits for couplingtheselector and the amplifiers in cascade, said link circuits'includingscreened grid tubes.

22. A radio circuit comprising'a selector having variable circuitconstants, a plurality of automatically tuned amplifiers having fixedcircuit constants and arranged to time over the same frequency range,and a plurality of link circuits for coupling the selector and theamplifiers in cascade, said link circuits including screened grid tubes.

23. A radio circuit comprising a selector having variable circuitconstants, a plurality of automatically tuned amplifiers having fixedcircuit constants and arranged to tune over the same desired frequencyrange, and a plurality of link circuits for coupling the selector andthe amplifiers in cascade, said link circuits including screened gridtubes, the last tuned circuit of the selector acting as the inputcircuit of the first screened grid tube, and the input circuits of theremaining screen grid tubes being naturally resonant to a constantfrequency lower than the desired range of frequency.

LESTER L. JONES. JACOB YOLLES.

